Engineering materials, particularly metals or metal alloys, exhibit widely varying properties based on the composition of the metal, the processing of the metal, and the method used in forming an article from the metal. The term "metal" as used herein will be used generally to mean metals and their alloys, unless otherwise indicated. Products made from metals are generally produced by casting or by forming. In general, forming is accomplished by the plastic working of a metal such as steel. Plastic working is the permanent deformation accomplished by applying mechanical forces to a metal. The primary objective of such working is usually the production of a specific shape or size, but may in some cases be done to achieve improved physical or mechanical properties. Plastic deformation of metals normally is accomplished by one of two methods: hot working and cold working. In hot working, the metal is heated to the proper temperature which is above the recrystallization temperature and then deformed. After deformation, the basic strength of the metal is essentially unchanged. Methods of hot working are hammering, pressing, rolling and extrusion. Hot working is generally performed for purposes of shaping.
Cold working is the deformation without heating or deformation done below the recrystallization temperature. Cold working may be used for the shaping of thin articles and is commonly used to achieve improved mechanical properties, better machinability, special size accuracy, and the production of thinner material than could be economically produced by hot working.
Engineering materials generally exhibit limited formability because of the plastic instability that occurs where hardening processes are inadequate to constrain localized inhomogeneous strains (necking) under developing tri-axial stresses. Many methods have been used to improve the formability of metals in efforts to increase resistance to necking, tearing or galling during forming processes. For example, U.S. Pat. No. 3,873,458 to Parkinson, discloses a lubricant coating containing a resin which is applied to metals during forming to lubricate the metal, allowing it to flow more uniformly during the forming process, thereby reducing the likelihood that it will tear or gall.
Many times scales or impurities on or near the surface of a metal will increase its necking characteristics thereby decreasing its workability. When certain metals such as alloys of titanium are heated during hot workings they react with the atmosphere increasing the amounts of contaminating impurities in those metals, thus limiting their workabilities. U.S. Pat. No. 3,339,271 to Durfee, et al. discloses a nickel plating for titanium alloys which serves to protect the alloy from reaction with the atmosphere during hot working, thereby improving its workability.
Certain metals exhibit the characteristic of superplasticity which permits those metals to be elongated from about 300 percent to about 1000 percent and allows complex forming processes to be accomplished with those metals. A method for superplastic forming was disclosed in U.S. Pat. No. 3,340,101 to Fields, Jr., et al. This patent discloses procedures for heating or otherwise conditioning a metal to produce the appropriate strain rate sensitivity followed by placement of the metal in an apparatus for forming. U.S. Pat. No. 3,934,441 to Hamilton, et al. discloses a method for the superplastic forming of titanium alloys in a controlled environment to prevent contamination of the alloy.
In general, high strength materials are brittle, i.e., having the tendency to fracture without appreciable deformation, and therefore exhibiting low ductility and malleability. Therefore, fabrication of complex high strength items requires expensive machining and fabrication techniques. Thus, although many high strength metals have been developed, numerous metallurgical and material processing problems exist which preclude their applications of those metals, to low cost, high strength structures such as the types desired for supersonic tactical missiles. Examples of such problems include forming and fastening, cracking at welds, susceptibility to hydrogen embrittlement, stress corrosion and cost effective heat treatments of the completed structural configurations. On the other hand, metals exhibiting ductility and malleability or which exhibit superplastic characteristics are generally of medium or low strength and while easily formed, do not possess the required strength for desired application. This is especially true in aeronautical applications where the strength to weight ratio is extremely important.
Thus, there has been a continuing need for a high strength material which may be formed providing production of economical and lightweight high strength structural configurations, especially in the aeronautical industry. The present invention provides a major advance in engineering materials. The present invention provides for a lamination of superplastic metals to non-superplastic high strength metals which produces a laminate that not only exhibits improved plastic or superplastic characteristics, but also high strength. This laminate may be formed by known superplastic forming methods to achieve a high strength formed structure.